High frequency component exhibiting magic t properties



March 26, 1968 T. s. SAAD 3,375,471

HIGH FREQUENCY COMPONENT EXHIBITING MAGIC T PROPERTIES Filed Nov. 6,1964 3 Sheets-Sheet 1 FIG. 2

INVENTOR. THEODORE s. SAAD ATTORNEYS March 26, 1968 T. s. SAAD 3,375,471

HIGH FREQUENCY COMPONENT EXHIBITING MAGIC T PROPE Filed Nov. 6, 1964RTIES 5 Sheets-Sheet 5 INVENTOR THEODORE S. SAAD ATTORNEYS March 26,1968 T. s. SAAD 3,375,471

HIGH FREQUENCY COMPONENT EXHIBITING MAGIC T PROPERTIES Filed Nov. 6.1964 5 Sheets-Sheet 5 I/VVE/VTOI? THEODORE S. SAAD MWMM United StatesPatent HIGH FREQUENCY COMPONENT EXHIBITING MAGIC T PROPERTIES TheodoreS. Saad, Westwood, Mass'-., assignor' to Sage Laboratories, Inc., East-Natick, Mass.-, acorporation of Massachusetts v Filed Nov. 6, 1964,.Ser.No. 409,421 11 Claims. (Cl; 333-11) ABSTRACT OF THE DISCLOSURE A cavityhas opposed generally parallel conducting end Walls for establishing anelectric field' generally 'per pendieular to the end walls. A conductingstrip-midway between and generally parallel to the end walls defineswith respective ones of the end-walls parallel plate transmission linesfor couplingenergy from a TEM input branch to intermediate branches inrespective ones of the end walls that are coupled to the conductingstrip. An odd mode branch is for establishingan odd'TE mode in thecavity having electric field components perpendicular to the end wallsso that when the branches are terminated in their respectivecharacteristic impedances, energy applied to one of the TEM or odd modebranches divides substantially equally between the intermediate brancheswithout reaching the other of the odd or TEM mode branches. Withcrystals terminating the respective intermediate branches a fifthterminal maybe provided with a lead to the conducting strip forwithdrawing energy at I-F frequency. In one embodiment of the inventionthe cavity is circularly cylindrical with-circularend walls and a linkcoupling from the odd mode branch to the cavity. In another embodimentof the invention the cavity is essentially a portion ofrectangularwaveguide with the end walls comprising the broad walls of such awaveguide, and the odd mode branch comprises a rectangular opening forreceiving energy froma" rectangular waveguide of corresponding crosssection.

The present invention'relates-in general to high frequency componentsexhibiting'magic T properties and more particularly concerns a novelcomponent especially useful in balanced mixing characterized by lownoise and excellent port isolation. Not only does the invention exhibitexcellent electrical properties, but it is'relatively easy andinexpensive to fabricate and embodies acom-' pact, rugged mechanicalarrangementwith an especially convenient arrangement of terminals.

A conventional waveguide magic T is'a' hybrid junction having fourintersecting waveguide branches. Typically three of these' branches arecoplanar and form a T in defining the H plane junction. The tourthbranchintersects the other three at right angles. It 'has hybrid'ipr'op ertiesof power division and isolation-and the property of being substantiallyreflectionless for a wave-propagating into the junction from any armwhen theOth'e'r'three arms are match terminated. While these propertieshave been achieved in coaxial devices. fa'br-ication of such devices hasbeen relatively costly and the arrangement'of terminals not asconvenient as desiredi Accordingly, it is an important object of 'th'i'sinven tion to provide a device having-magic- T' properties-that may beusedto intercouplea plurality'ofcoaxial transmission lines while keepingfabrication costs low and providing an exceptionally convenientarrangement of ice balanced mixer in accordance with the precedingobjects' characterized by low noise properties.

According to the invention, conducting means define a cavity. Meansdefine a TEM branch responsive to a potential applied at its input forestablishing the TEM mode in the central region of the cavity andcoactin'g with'first' and'second opposed wallport'ions of the cavitydefining conducting means to define first and second two conductortransmission lines respectively between the TEM branch input and thecavity central region.

First and second intermediate branches are arranged for couplingexternal devices to the central region and are respectively coupled tothe TEM branch input by the first and second two conductor transmissionlines, respectively. The first and second two conductor transmissionlines are substantially the same electrical length. Means define an oddmode branch responsive to a potential applied at its input forestablishing an odd TE mode in the central region of the cavity so thatwhen the first and second intermediate branches are terminated in equalimpedances, energy of a prescribed frequency applied to the odd modebranch is transmitted through the cavity to the intermediate branches insubstantially equal amounts but of opposite electrical phase.

In a specific form of the invention the conducting means defines ahollow circular cylinder having first and second circularconducting endplates separated by aldistance less than the cylinder diameter; The TEMbranch input comprises a coaxial terminal pair in the curved conductingwall of the cavity defining conducting means with the terminal pairinner conductor being coupled to the central region 'by means includinga con-' ducting str'i'p generally parallel to and midway between thecircular conducting end plates. The first and-second intermediatebranches each comprise" coaxial terminal pairs inre'spective ones of thefirst and second circular endplates with the inner conductor of eachpair being coupled to the conducting strip in the central portion. Theodd mode branch may comprise a coaxial terminal pair in thecurved'co'nducting wall with a link coupling the inner terminal of thelatter pair to one of the'end plates.

Numerousother features, objects and advantages of the invention willbecome apparent from the following specification when read in connectionwith the accompanying drawing in which: 7

FIG. 1 is a top view of an embodiment of the invention with the upperend plate removed;

FIG. 2 is an axial sectional view of an embodiment o the inventionembracing section 22 of FIG. 1;

FIG. 3 is a top view with upper end plate removed of a variation of theembodiment of FIGS. 1 and 2 with the intermediate branch connections tothe conducting strip staggered;

FIG. 4 is a view essentially thru section 4:4 of FIG. 3 but showingadditional detail; and

FIG. 5 is a sideview of'an embodiment ofthe invention for mixing energyprovided by a rectangular waveguide;.

FIG. 6-is an end' view of the embodiment of FIG. 5 aswo'uld beseen by.an input rectangular waveguide;

FIG. 7 is a viewthrough section 7-7 of FIG. 5; and

FIG. 8 is a view through section 8'8 of'FIG. 6.

With reference now to the drawing, there is shown an especiallyadvantageous embodiment of the invention inv which the sides andbottomof the cavity defining means are formed by turning a solid conductingcylinder to form an annular wall with the desired inside diameter whichtypically corresponds to a half wavelength of a frequency within theoperating range of the device; Thus, the bottom circular endplate 11- iscontiguous with the curved wall 12 surrounding the cylindrical cavityaxis embraced by the upper intermediate branch inner terminal 13 and thelower intermediate branch inner terminal 14. An upper circularconducting endplate 15 completes the definition of the cavity.

The coaxial terminal pair 16 at the right, comprising an outer conductor17 and an inner conductor 18 partially surrounded by an annulardielectric bead 21, functions as the input of the TEM branch. This TEMbranch also comprises the conducting strip 22 midway between bottomendplate 11 and top endplate 15. Strip 22 coacts with these endplates toform upper and lower parallel plate transmission lines for couplingenergy between the T EM input 16 and the central region of the cavity.The central region includes upper dielectric bead 23, coaxiallysurrounding the upper intermediate branch inner conductor 13, and lowerdielectric bead 24, coaxially surrounding the lower intermediate branchinner conductor 14. The junction of these two conductors is connected toinner conductor 18 by strip 22. Inner conductors 13 and 14 are coaxiallysurrounded by upper outer conductor 25 and lower outer conductor 26,respectively, outside the cavitydefining casing to define upper andlower coaxial terminal pairs..

The left coaxial terminal pair 27 functions as the input to the odd modebranch and comprises an inner conductor 31 coaxially surrounded by anouter conductor 32. An L-shaped conductor 33 is connected from innerconductor 31 to a point 34 in bottom plate 11 to coact with theconductive casing and define a loop for effecting loop coupling betweenthe coaxial terminal pair 27 and the inside of the cavity so as toestablish an odd TE mode in the cavity. The tuning screws 35 in theupper end plate 15 above the elbow of conductor 33 coact therewith toadjust the effective capacity so as to improve the match presented atthe coaxial terminal pair 27.

Turning now to the consideration of what are believed to be theprinciples of operation of the invention, it is convenient initially toassume that upper and lower intermediate branches are terminated inequal impedances. Then energy applied to the TEM input 16 establishesthe field represented by vectors 36 and 37 when inner conductor 18 ispositive with respect to the conductive casing. The energy transmissionpaths above and below conducting strip 22 are of substantially equalelectrical length so that the energy divides and reaches innerconductors 13 and 14 in equal amounts and substantially in phase. Theenergy reaching that portion of the cavity embracing the link 33 wouldseem to establish an electric field con sisting of vector componentsoriented like vectors 36 and 37, indicated as 36 and 37, ofsubstantially equal magnitude but opposite phase so that virtually nopotential is established between inner and outer conductors at branch27.

Now consider the situation in which energy is applied to the odd modebranch 27. This is believed to establish an electric field representedby the vector 41 which may be broken down into an upper component 42 inthe upper half of the cavity and a lower component 43 in the lower halfof the cavity of substantially equal magnitude to that established inthe upper half so that the upper and lower intermediate branches receiveenergy of substantially equal magnitude but of opposite phase for theinner conductor 13 will then be negative with respect to the conductivecasing while the conductor 14 will be positive. Very little energypasses to the right to reach the TEM branch because the field at thatpoint is oriented in the direction of vectors 42 and 43, as indicated bythe vectors 42' and 43', so that the resultant field magnitude betweenthe conducting plate 22 and the conductive casing is substantially zero.This mode of operation is believed to contribute largely to theproperties of the device having high isolation over a relatively widerange of frequencies.

Referring to FIG. 3, there is shown a variation of the embodiment of theinvention shown in FIGS. 1 and 2 with the connections from theintermediate branches to the conducting strip and a fifth terminal pairprovided for withdrawing intermediate frequency energy produced bymixing energy applied at the TEM branch and the odd mode branch. Whereapplicable, the same reference symbols identify corresponding elementsthroughout the drawing. The view of the structure seen in FIG. 3 issimilar to the view seen in FIG. 1 with a few exceptions. In thestructure of FIG. 3, especially advantageous for balanced mixing, theconnections to conducting strip 22 at 51 and 52 are side-by-side along adiameter at right angles to the common axis of TEM input terminal pair27. A dielectric support bead 53 is displaced from the connection points51 and 52. A thin lead 54 connects the end of conducting strip 22 nearconnection point 52 to an output signal inner coaxial terminal 55located in the outside wall 12 for delivering intermediate frequencyenergy to an external circuit. The outline of lower crystal 56 isvisible in the view of FIG. 3.

Referring to FIG. 4, there is shown a sectional view essentially throughsection 44 of FIG. 3 to illustrate the crystals and supporting structureand the intermediate frequency output terminal in greater detail. Themeans for supporting lower crystal 56 and establishing the desiredelectrical connections is shown in detail in FIG. 4. The crystal holderincludes a threaded sleeve 57 formed with an annular flange 61 abuttingthe bottom of the lower end wall 11 and force fit therein. A cup-shapedsleeve 62 formed with a shoulder 63 that rests against the top ofconducting strip 22 has its narrow diameter end force fit in the openingof connection point 52 and has its upper end resting firmly against thepiece of thin mylar insulating tape 64. A spring 65 of conductingmaterial establishes firm electrical and mechanical contact with innerterminal 66 of lower crystal 56. The resulting structure establishes onecontact between the crystal and conducting strip 22, helps to supportthe conducting strip 22 and maintains the integrity of the insulatingrelationship between the outside walls defining the cavity and theconducting strip 22.

Referring to FIGS. 5-8, there is illustrated an embodiment of theinvention arranged for mixing a signal applied at the TEM coaxialterminal pair 71 with a signal provided at the odd mode branch 72,arranged to mate with a rectangular waveguide, to provide anintermediate frequency signal on output terminal in response to themixing action effected by crystals 73 and 74 terminating intermediatebranches 75 and 76, respectively. The crystal holders comprise anassembly of wall known design including crystal current monitoring caps.

The portion 77 of conducting strip 78 near the center of the cavity isconnected to first and second diode contact receptacles 81 and 82,respectively, for receiving contacts 83 and 84 of diodes 73 and 74,respectively. First and second insulating sleeves 85 and 86, which maybe of rexolite material, surround diode contacts 81 and 82, respectivelyand diodes 73 and 74, respectively and help support conducting strip 78.

The intermediate frequency coaxial signal terminal 70 is supportedwithin an insulating sleeve 107 which in turn carries an annularconducting sleeve 108. A thin conducting wire 111 connects the nearestcorner of conducting strip 78 to the inside end of the IF terminal 70.The latter end is bypassed to ground by inductor 112. A tuning post 113is located near the corner 114 of conducting strip 78 to adjust theeffective reactance so as to improve the match presented at thewaveguide terminal 72.

The particular structure of FIGS. 5-8 forms an especially convenientmeans for mixing an input signal provided by a waveguide with a localoscillator signal provided over a coaxial cable in a compact physicalconfiguration providing a convenient means for deriving the outputsignal .while maintaining exceptionally high electrical performance.

An embodiment of the type disclosed in FIGS. 5-8 operates over thefrequency range of 3,600 to 4,200 megacycles using type 1N416 crystalsto provide an intermediate frequency of 70 megacycles with a noisefigure of 8 db while maintaimng an isolation of greater than 25 dbbetween local oscillator and signal ports.

In an exemplary embodiment of the invention actually constructed of thetype shown in FIGS. 1 and 2 for operation over a frequency range of 5.4gigacycles to 6.0 gigacycles, the four coaxial terminal pairs wereconventional type N connectors, the inside diameter of the cavity was1.6", the inside height of the cavity was /2", the diameter of theannular Teflon dielectric beads 23 and 24 was 0.4, the width of strip 22was .550", its length to the constricted portion was .750", and point 34was located .440" from the inside wall.

'With the upper and lower intermediate branches terminated in coaxialcrystal holders with type 1N23E crystals and TEM input 16 energized withlocal oscillator signal of 2 mw. and at 6,000 me. and odd mode input 27energized with the signal input at 6,030 mc., an intermediate frequencysignal at 30 me. was provided with a noise figure of 6.7 db whilemaintaining an isolation between signal and local oscillator ports of22.5 db.

The first exemplary embodiment of the invention describedherein embodiesa circular cylindrical. cavity and includes loop coupling to establishthe odd mode. Other cavity shapes, such as rectangular, elliptical andspherical may be employed within the principles of the invention. Othermeans, such as probe coupling may also be employed to establish the oddmode. The specific embodiments and uses described herein are by way ofexample only. It is apparent that those skilled in the art may now makenumerous modifications and uses of and departures from the specificembodiments described herein without departing from the inventiveconcepts. Consequently, the invention is to be construed as limited onlyby the spirit and scope of the appended claims.

What is claimed is:

1. A high frequency component exhibiting magic T properties comprising,

conducting means defining a cavity having first and second parallelopposed end wall portions, means defining a TEM branch responsive to apotential applied at its input for establishing the TEM mode withelectric field perpendicular to said end Walls in the central region ofsaid cavity and including a conducting strip midway between, parallel toand coacting with said first and second opposed end wall portions ofsaid conducting means to define first and second two conductortransmission lines respectively between said TEM branch input and saidcentral region, said end wall portions separated by substantially thesame distance throughout said cavity,

first and second intermediate branches in respective ones of said endwall portions for coupling external devices to said central region andrespectively coupled to said TEM branch input by said first and secondtwo conductor transmission lines respectively, first and second leadsperpendicular to said first and second end wall portions coupling saidfirst and second intermediate branches respectively to a point on saidconducting strip substantially at the center of said cavity, said firstand second two conductor transmission lines being of substantially thesame electrical length,

and means defining an odd mode branch responsive to a potential appliedat its input for establishing an odd TE mode in said central region sothat when said first and second intermediate branches are terminated inequal imped-ances, energy of a prescribed frequency applied to said oddmode branch is transmitted through said cavity to said intermediatebranches in substantially equal amounts but of opposite electricalphase.

2. A high frequency component exhibiting magic T properties inaccordance with claim 1 wherein,

said conducting means defines a hollow circular cylinder having firstand second circular conducting end plates separated by a distance lessthan the cylinder diameter, said TEM branch input comprises a coaxialterminal pair in the curved conducting wall of said conducting meanswith the terminal pair inner conductor being coupled to said centralregion by means including said conducting strip generally parallel toand midway between said circular conducting end plates,

said first and second intermediate branches each comprise coaxialterminal pairs,

said first and second intermediate branch pairs being in said first andsecond circular end plates with the inner conductor of each pair beingcoupled to said conducting strip in said central portion.

3. A high frequency component exhibiting magic T properties inaccordance with claim 2 wherein,

said odd mode branch comprises a coaxial terminal pair in the curvedconducting wall opposite to said TEM branch input coaxial terminal pairand a link coupling the inner terminal of the latter pair to one of saidend plates.

4. A high frequency component exhibiting magic T properties inaccordance with claim 1 and further comprising,

first and second unilaterally conducting devices terminating said firstand second intermediate branches respectively,

and an output terminal coupled by a lead extending through andinsulatedly separated from said cavity defining conducting means to saidcentral region for extracting energy from said central region.

5. A high frequency component exhibiting magic T properties inaccordance with claim 2 and further comprising,

first and second unilaterally conducting devices terminating said firstand second intermediate branches respectively,

and an output terminal extending through and insulatedly separated fromsaid cavity defining conducting means coupled to said conducting stripnear the end thereof in said central region.

6. A high frequency component exhibiting magic T properties inaccordance with claim 5 wherein,

said odd mode branch comprises a coaxial terminal pair in the curvedconducting wall opposite to said TEM branch input coaxial terminal pairand a link coupling the inner terminal of the latter pair to one of saidend plates.

7. A high frequency component exhibiting magic T properties inaccordance with claim 1 wherein,

said odd mode branch comprises means defining a rectangular opening forengaging a rectangular waveguide.

8. A high frequency component exhibiting magic T properties inaccordance with claim 7 and further comprising,

first and second unilaterally conducting devices terminating said firstand second intermediate branches respectively,

and an output terminal coupled by a lead extending through andinsulatedly separated from said cavity defining conducting means to saidcentral region for extracting energy from said central region. 9. A highfrequency component exhibiting magic T properties in accordance withclaim 8 wherein,

said conducting means defines a hollow rectangular waveguide cavityhaving broad and narrow wa-lls,

said TEM branch input comprises a coaxial terminal pair in one of saidnarrow walls with the terminal pair inner conductor being coupled tosaid central region by means including a conducting plate generallyparallel to and midway between said broad walls,

said first and second intermediate branches each comprise coaxialterminal pairs,

said first and second intermediate branch pairs being in respective onesof said broad walls with the inner conductor of each pair being coupledto said conducting strip in said central portion.

10. A high frequency component in accordance with claim 9 wherein saidcavity defining means includes an end wall generally perpendicular tosaid broad and narrow walls and said output terminal extends through andis insulatedly separated from the latter end wall and coupled to saidconducting plate near a corner thereof in said central region near saidlatter end wall.

11. A high frequency component exhibiting magic T properties inaccordance with claim 7 wherein,

said conducting means defines a hollow rectangular waveguide cavityhaving opposed broad walls perpendicular t0 opposed narrow walls,

said TEM branch input comprises a coaxial terminal pair in one of saidnarrow walls with the terminal pair inner conductor being coupled tosaid central region by means including said conducting plate generallyparallel to and midway between said broad walls,

References Cited UNITED STATES PATENTS 11/1951 Rodwin 33311 X 5/1960Dwork 333-41 11/1962 Whitehorn 333l1 X 12/1962 Stracca 33311 FOREIGNPATENTS 5/ 1956 Germany.

HERMAN KARL SAALBACH, Primary Examiner. 20 ELI L'IEBERMAN, Examiner.

R. D. COHN, S. CHATMON, Assistant Examiners.

